The invention relates generally to fueling of compressed gas tanks, and more particularly, to methods and apparatus for safe fueling of compressed gas tanks.
A key component in a high pressure storage system for vehicle applications, such as hydrogen, or compressed natural gas (CNG), is the tank vessel. One type of tank vessel for storing compressed gas is a made of a fiber composite. Fiber composite vessels are desirable because they have a good storage to weight ratio. They typically have two layers: an outer layer, made of a carbon fiber matrix for example, that is designed to bear the mechanical load, and an inner layer, or liner, made of a bubble of plastic or aluminum, that is designed to prevent leaking.
The geometry of the inner liner and the fiber matrix layer are usually different for material and process reasons. The inner layer generally cannot withstand stress forces. To ensure that the liner is firmly supported by the outer layer, a minimum pressure should be maintained at all times. At pressures below the minimum pressure, for example less than about 20 bar, the two layers may separate from each other. If fuel is introduced into the vessel quickly under high pressure below the minimum pressure, the inner liner will bump against the fiber matrix layer very hard. The liner could rupture, and the contents would flow through the outer layer into the environment. In addition, gas trapped in the gap between the two layers can damage the liner and/or the fiber matrix layer.
Tank pressure evaluation has a certain tolerance, which increases over time. For example, the tolerance chain is the sum of several components and the converter. The tolerance for the P-transducer includes the A/D converter, the temperature compensator, and the D/A converter, which could be ±13.5 bar, for example. The tolerance for the vehicle controller could be ±18 bar, for example. Thus, the signal tolerance would be 31.5 bar, for example. The degradation over time needs to be considered also, which could be ±2.25 bar/yr, for example. Thus, for a 4 year old p-transducer, the tolerance would be the sum of these, or ±40.5 bar, for example.
The worst case tolerance margin has to be added to the signal tolerance along with a safety margin. Thus, for reasons of safety, hydrogen release may need to be stopped at a tank pressure of 65 bar by the vehicle controller (e.g., 20 bar minimum pressure+31.5 signal tolerance+9 bar degradation+4.5 bar safety margin=65 bar). However, at the calculated shutdown pressure, the “real” tank pressure could be any value between 20 bar and 105 bar, as shown in FIG. 1. Therefore, in all non-worst case situations, the worst case tolerance margin results in a reduced usable hydrogen mass and thus a reduced vehicle range of up to 14%.
Typically, the driver of the vehicle fuels at a filling station. Some fueling stations have the ability to recognize the vessel filling grade, while others do not. If the fueling station has an infrared (IR) interface, a computer, and mapping, the fueling station can control the proper fueling. The vehicle storage tank is connected to the filling nozzle of the station and creates a “short cut” between the empty vessel and the 875 bar pressure of the station. The “short cut” between the empty vessel and the 875 bar source creates a very strong pressure jump and very high gas flow. A slow fill has a reduced flow compared with the regular fast fill. Presently, slow fill is only possible at a service hub with trained personnel and special equipment (H2 bottle, flow restrictor, pressure gauge).
Furthermore, it is believed that fast filling has been done improperly in some situations, e.g., without knowledge, without tracing, and/or without instrumentation.